1. Field of the Invention
The invention relates to the field of color management, and in particular, to CMYK to CMYK color conversion of image data in a manner that preserves the gray color values of the image data upon color conversion.
2. Statement of the Problem
In color management, a device may convert colors of input image data to a color space of an output device. For example, the printer may have a CMYK color space that is used to represent the image data using various levels of Cyan (C), Magenta (M), Yellow (Y), and blacK (K). Before the image data is printed, the input image data is converted to the gamut of the CMYK printer. However, precise representation of the image data is generally not possible as information is lost or misrepresented in the conversion process. Other devices that may perform color conversion include monitors, printers, cameras, and scanners.
In certain instances, color conversion is performed between like color spaces, such as CMYK to CMYK. Such a color conversion may appear to be trivial, however, the process generally requires conversion from an input CMYK color space to a perceptual color space (e.g., CIEL*a*b*) and then to the output CMYK color space. The CIEL*a*b* color space is visualized as three dimensional color space, where every color that humans can see is uniquely located. Though the CIEL*a*b* color space is a perceptual color space, it is not a perceptually uniform color space as the Euclidean distance in the space does not correspond to the perceptual distance. For example, the magnitude of the perceptual color difference generally depends upon the color location and the changing direction in chroma and hue. CIEL*a*b* increasingly overstates the magnitudes of perceived chroma differences. In general, the human visual system (HVS) is sensitive to the color change in the neutral color area and relatively insensitive to the color change in a highly saturated color area. The CIEL*a*b* color space is also non-uniform regarding hue angle in that the thresholds of visual tolerances are a function of hue angle. If the non-uniformity of the CIEL*a*b* color space in chroma and hue is examined from another perspective, CIEL*a*b* colors have different characteristics at different locations.
A color conversion model can be generated using this data to convert CMYK color values to the CIEL*a*b* and vice versa. Generally, the relationship between CMYK color space and the CIEL*a*b* color space is nonlinear due to the interactions of cyan, magenta, yellow, and black planes, leading to more complex color conversion processing. In a color management system, color profiles that describe the color characteristics of certain devices or color spaces are used in converting color image data from a device-independent color space (e.g., CIEL*a*b*) to a device-dependent color space (e.g., CMYK) or vice versa. A color management system may include color profiles that provide information for converting color image data from device-dependent color spaces, such as CMYK or RGB, to device-independent color spaces, such as CIEXYZ or CIEL*a*b*, and back
A color profile has a particular structure that is specified in International Color Consortium (ICC) standards. Within each profile is a rendering intent that establishes the intended usage of the image data (e.g., printing, displaying on computer monitor, etc.). There are four types of rendering intent: the relative colorimetric; absolute colorimetric; perceptual; and saturation. In printing, a printer's saturation rendering intent is generally used to brighten colors and make them stand out on a printed medium, whereas the perceptual rendering intent is intended to present more esthetically pleasing images.
To print CMYK jobs with preserved K in a CMYK to CMYK color conversion, a certain K color value of the input color space wherein CMYK=(0, 0, 0, K) is mapped to a K color value of the output color space C′M′Y′K′=(0, 0, 0, K′). The remaining color values are converted via standard color management. The CMYK to CMYK color conversion, however, is not available for certain types of image data processing. Examples include PostScript and PDF processing, where the color space array (CSA) and the color rendering dictionary (CRD) are merely input and output ICC profiles, respectively. One manner of solving this problem includes modifying an output CMYK profile to map “achromatic” colors to flat K color values; however, this solution generally loses valuable color information in the conversion process, causing some color values to remain as full CMYK color values instead of being forced to K color values as desired. Thus, there is a need to preserve the K color values during a CMYK to CMYK color conversion.